1. Field of the Invention
This invention relates generally to fuel cells, and more particularly, to means for mechanically retaining a plurality of electrically connected fuel cells in a stack, more specifically to a means for applying compressive loading to the stacked fuel cells.
2. Description of the Prior Art
A fuel cell basically comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed between the two electrodes. Typically, each electrode may comprise a thin catalyst layer adjacent to the electrolyte and disposed upon a layer of support material or substrate. A reactant gas compartment is behind the substrate, and the substrate is porous to gas in a direction perpendicular to its opposite faces, i.e., across the thickness dimension of the plate, so that reactant gas may diffuse therethrough to the catalyst layer. An electro-chemical reaction occurs at the gas/electrolyte/catalyst interface whereby ions travel from one electrode to the other through the electrolyte, producing useable electrical energy.
In order to obtain commercially useful amounts of electric power, it is necessary to stack a plurality of cells and connect them electrically in series. Electrically conductive, gas-impermeable plates usually separate the anode of one cell from the cathode of the next adjacent cell. The voltage across the stack is the sum of the voltage drops across the individual cells and is a function of the current produced by each cell. Further, the amount of current produced by each cell is directly proportional to the amount of reactant gas utilized in the electro-chemical reaction.
An important factor affecting the performance of fuel cells is the clamping pressure exerted upon the cells as they are held together in the assembled stack. For instance, the ion diffusion and the electro-chemical action of the cell is significantly affected by only small changes in contact pressure between the electro-chemically active materials of the cell. Recognition of this fact is disclosed in U.S. Pat. No. 2,594,713, which shows in FIG. 1 thereof a test apparatus for analyzing the effects on the performance of the cell stack of pressure variation between the cells.
U.S. Pat. Nos. 3,012,086, 3,253,958, 3,356,535, 3,444,714 and 3,982,961 all disclose various fuel cell stack constructions in which a plurality of cells are held together in assembled relationship by a plurality of tie bolts or rods. Additionally, U.S. Pat. No. 3,253,958 discloses a resilient follow-up means for compensating for stack compression. U.S. Pat. No. 3,232,950 discloses a fuel cell construction in which a fuel electrode 2 and an oxidant electrode 3 are in intimate contact with opposite sides of an electrolyte carrier or disk 1. The electrodes are urged against the electrolyte disk by means of springs 13 and 14 engaged against the electrodes and end plates or caps 17 and 18. U.S. Pat. No. 949,619, not concerned with fuel cells, discloses a box structure in which tension springs engage lids to retain them on the box.
Conventional fuel cell stack designs are seen to employ tie-rods and compressive loading elements to initially apply and later maintain the required cell compressive loading to the cell stack. As exemplified by the prior art described above, these compressive loading elements are typically disposed outboard of the top and bottom end plates of the fuel cell stack. One typical conventional construction utilizes tie-rod extensions projecting beyond the end plates with helical compression springs fitted over the extensions and compressed by means of equally torqued locking hex-nuts.
Such conventional prior art constructions suffer from several disadvantages. For instance, a major disadvantage of such designs is in the limited load follow-up capability. Since the spring rate is high, cell compression significantly decreases as the stack shrinks during operation of the stack and periodic retensioning of the compressive loading elements is necessary. The impact of shrinkage of the cell stack may be counteracted by selected longer springs. However, this leads to increased stack height, since the compressive load elements are disposed outboard of the end plates in conventional structures.
Another known construction utilizes tie-rods placed externally of the reactant manifolds and operatively connected with parallel bars placed over the end plates to exert a clamping force to the end plates. In this configuration, the bars function essentially as leaf springs. This structure yields a low stack height but requires a larger effective cross-section area of the assembled cell stack. In addition, a leaf spring configuration of this type has a limited spring excursion capability which may not be sufficient to take up the full extent of stack shrinkage.